Recently, x-ray lithography has been used in certain semiconductor wafer processing operations to replace the older ultraviolet (UV) photolithographic processes in order to improve the yield and resolution of replicated patterns relative to the yield and resolution achievable with these earlier UV processes. Certain types of these x-ray lithographic processes for mask alignment are described, for example, in copending application Serial No. 682,432 now U.S. Patent No. 4,085,329 and in U.S. Pat. No. 4,019,109, both assigned to the present assignee. These prior disclosures and the references cited therein are incorporated fully herein by reference.
The improved yield which may be realized by the use of x-ray lithography in place of contact UV photolithography results in part from the fact that in x-ray lithography the mask and wafer need not be in direct contact with each other during exposure of the wafer. Thus, this non-contact feature produces higher yields than those produced by direct contact printing techniques. Additionally, there is no loss of resolution for the reason that x-rays do not scatter or diffract as does UV light when a conventional photolithographic mask is physically separated from the semiconductor substrate with which it is aligned. Additionally, short wavelength x-rays pass through dust and low atomic number contamination without the absorption and dispersion that is produced using ultraviolet radiation. Furthermore, the non-contact aspect of x-ray lithographic processes results in a much longer mask life relative to those lithographic processes which require the mask to be in direct contact with the semiconductor wafer.
One disadvantage of using x-ray lithography in place of photolithography is the time required for x-rays to properly expose patterns in resist. In order to reduce this x-ray resist exposure time, consideration has been given to either increasing the power output of the point source of x-rays or moving the x-ray source closer to the wafer on which the resist patterns are developed, or both, since either of these modifications will have the effect of reducing the resist development time. However, by bringing the source of x-rays closer to the wafer being processed, an increased magnification is introduced into the pattern projected from the mask onto the wafer. Such magnification effect is produced when the x-rays pass through off-center areas of the mask at an increasing angle from the normal as the distance between x-ray source and wafer is reduced. This magnification effect can in turn produce intolerable variations in the location of patterns projected onto the wafer surfaces unless the opposing mask and wafer surfaces lie in precisely parallel planes with a uniformly reproducible gap between them. This problem is particularly significant, for example, where the wafer surface is bowed or wavey.
A second undesirable effect of mask-to-wafer spacing variations in x-ray lithography is the penumbral shadow which results when the edge of a feature on the mask is projected onto the wafer with x-rays from a finite size source. This penumbral shadow is a transition region across the edge of a feature where the exposure dose received in the resist gradually changes from full exposure to no exposure. The width of this penumbral shadow is directly proportional to the width of the x-ray source and to the gap between the mask and wafer and is inversely proportional to the distance from the x-ray source to the mask. Variations in the mask-to-wafer gap produce variations in the width of this penumbral shadow, and this results in undesirable variations in the size of features produced in the x-ray resist when it is developed.
Thus, as will become more readily apparent herein, a constant distance between mask and wafer across the entire facing surfaces of both of these members is manifestly desirable in order to achieve minimum distortion in the replicated micropattern and a minimum of undesirable variations in the resist patterns being developed.
In U.S. Pat. No. 3,743,842 issued to Henry I. Smith et al there is shown one method for providing a separation between mask and wafer during an x-ray lithographic process. This method utilizes an annular spacer which may be a part of the x-ray absorption mask through which the x-rays pass to reach the semiconductor wafer. However, the spacing techniques disclosed in U.S. Pat. No. 3,743,842 do not alleviate the problem of variable distances between mask and wafer throughout their surface areas, such as in situations described above where the wafer is wavey or bowed or both. It is the solution to this problem to which the present invention is directed.
In the IEEE Transactions on Electronic Devices, Volume ED-22, No. 7, July 1975 at page 421, Bernacki and Smith propose the use of studs on a photoresist mask to control the mask-to-substrate gap by forcing the wafer substrate into contact with the studs on the mask. These studs would be located within the kerf that separates individual microelectronic circuit patterns on the mask, and these studs might be 10 microns high and 25 microns in diameter, for example, and located at the corners of the individual microelectronic circuit patterns on the mask. That is, they would be located in a square array with a spacing of typically 1 to 10 millimeters. Additionally, the small studs proposed by Bernacki and Smith were to be located at the corners of the photoresist mask instead of ribs running along the kerf lines thereof, the reason being that this latter proposed configuration provides a smaller contact area and consequently a low probabililty that a stud would come to rest upon a dust particle or other defect. However, a significant disadvantage of this Bernacki and Smith approach will likely be encountered in actual practice when the studs begin to pick up resist from the substrate, and consequently the photoresist mask will require frequent cleaning, which may produce defects on the mask.